Fig. 4 (a) shows the stress-strain relationships on constant suction during shearing where α=45o, b =0.5, pnet =100kPa and different values of matric suction, s. When the matric suction is larger, the initial stiffness is steeper, and especially the difference between saturated specimen (s =0kPa) and unsaturated specimens (s > 100kPa) is quite large. The strain softening is occurring on every unsaturated specimen, which is different from the case of saturated one. The peak (failure) points come out at smaller shear strain, εs , when the matric suction is larger. It indicates that the soil specimen becomes brittle with with an increase in matric suction.

Fig. 4 (b) shows the volumetric strain-shear strain relationships in the same case shown in Fig. 4 (a). The unsaturated specimens start dilation around the failure points. When the matric suction is larger, the volumetric strain, εv , decreases, i.e., a more dilative response is observed.

A similar tendency is indicated in the results of different pnet conditions, which are shown in Fig. 5 (pnet =200kPa) and Fig. 6 (pnet =300kPa). The stiffness increasing with matric suction is observed at any pnet . It indicates that suction has an effect on increasing the initial stiffness.